The eye does most of its work at the end of a line, where it has to leap back and find the start of the next one. Give it a short trip and enough white space to land on, and it never asks you for help. Make the line long and tight, and the eye starts missing, rereading the same row or skipping a row entirely. You will not notice this happening. You will only notice that you are tired, and that the article you meant to finish has somehow become a thing you will read later.